Receiver volume range expansion circuit



March 29, 193& D. E. FOSTERY 2,112,278

RECEIVER VOLUME} RANGE EXPANSION cmcurr Filed Nov. 2a, 1956 AMPZ.

VON/ME Z-X AA DER mum/m INVENTOR DUDLEY FOSTER m 8 ATTORNEY Patented Mar. 29, 1938 UNITED STATES PATENT OFFICE CIRC Dudley E. Foster, Morristown,

Radio Corporation of America,

of Delaware UIT N. J., assignor to a corporation Application November 28, 1936, Serial No. 113,101

8 Claims.

My present invention relates to volume control circuits for radio receivers, and more particularly to a method of, and means for, automatically expanding the volume range of a receiver.

In the past it has been proposed to expand the volume range of a receiver to compensate for the inherent compression action taking place at the transmitter during the process of monitoring. Further, expander circuits were proposed to re ulate the receiver in a signalling system deliberately employing compression at the transmitter. In such prior arrangements the expansion control voltage is usually applied to an audio amplifier provided with a special gain control electrode. This is done to avoid distortion which arises when the signal grid of an audio amplifier is controlled for gain regulation.

It may, therefore, be stated that it is one of the main objects of this invention to provide a volume range expander circuit for a radio receiver wherein the gain control voltage is applied to a conventional signal amplifier feeding the detector without distortion resulting in the audio output of the receiver.

In automatic volume range expansion systems it is necessary to maintain the signal energy input to the controlled stage substantially uniform so that volume expansion will depend solely on percentage modulation rather than on carrier amplitude. It may, accordingly, be stated that another important object of my invention is to provide a volume range expander circuit for a receiver, wherein the signal carrier amplitude at the input circuit of the signal amplifier under expansion control is maintained substantially uniform by an automatic volume control arrangement deriving its signal energy from a point in the receiving system preceding the aforesaid amplifier input circuit.

Another object of my invention is to provide a relatively simple and effective type of expansion circuit for a receiver wherein a portion of the detected signal energy may be rectified, and the rectified energy used to increase the gain of a signal amplifier feeding the detector by decreasing the signal grid bias of the amplifier.

Still other objects of the invention are to improve generally the efiiciency and construction of receiver volume range expander circuits, and more especially to provide an expander circuit which is not only reliable and dependable in operation, but is economically manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown that portion of a superheterodyne receiver necessary for a proper understanding of the invention. It will be understood by those skilled in the art of constructing signalling systems, that the superheterodyne receiver shown will comprise the usual signal collector, such as a grounded antenna circuit, followed by one, or more, tunable radio frequency amplifiers; a tunable first detector circuit having amplified signals impressed thereon. A tunable local oscillator circuit feeds energy of proper frequency to the first detector circuit, and the usual uni-control tuning adjusting means will be employed to tune the signal and oscillator circuits through frequency ranges such that the best energy produced in the first detector output circuit will be of the desired 1. F. The latter may be chosen from a range of 75 to 4.80 kc., where the signal range is the broadcast band of 450 to 1500 kc. These circuits are well known, and are not shown in the drawing.

The I. F. energy is impressed on one, or more, I. F. amplifiers; the I. F. tuned transformer l impressing the signal energy upon the amplifier 2. The transformer 3 transmits the amplified I. F. energy to the following amplifier 4. The primary circuit of transformer 3 is resonated to the operating I. F., and the same is true of the secondary circuit 6. The tuned input circuit 6 is connected between the signal grid and cathode of amplifier 4; the resistor-condenser network 1 in the grounded cathode lead developing a direct current voltage to establish the normal bias of the signal grid 8.

The transformer 9, having its primary and secondary windings each tuned to the operating 1. F., couples the output electrodes of the amplifier 4 to the electrodes of the diode second detector. The tube 10 is provided with a cathode I! and a pair of anodes I2 and I3; it may be a 6H6 tube with both cathodes at ground potential, or it may be of the 55 type. If desired, separate diodes may be employed since the functions performed by the electrodes of tube Ill may be readily duplicated by other tube arrangements. The second detector stage comprises the tuned input circuit 9 connected in series with the load resistor I 4 between the anode l2 and grounded cathode ll resistor l4 being shunted by the I. F. bypass condenser I5.

The audio voltage component of the detected I. F. current flowing through resistor I4, is impressed upon one, or more, audio amplifiers, and a reproducer of any desired type can be used to reproduce the modulation on the carrier. To regulate the audio volume range of the receiver, there is provided the expander circuit now to be described. A portion of the audio voltage 'developed at resistor I4 is impressed on the signal grid I6 of rectifier tube I! by the condenser IS; the latter has a low impedance to audio currents. The tube I! has its plate connected to a source of positive voltage (not shown); the condenser l9 by-passing currents of audio frequency to ground. The cathode of tube I! is connected to ground through resistor 20; the latter being shunted by condenser 2 I. The resistor 22 is connected to ground from grid I6; it serves to provide a direct current path between grid l6 and the grounded side of bias resistor 20.

The direct current voltage developed across resistor 20 is dependent in magnitude upon the percentage modulation of the received carrier. The greater the amplitude of the audio voltage applied to grid l6, the larger is the direct current voltage developed across resistor 20. The latter voltage is applied to the signal grid 8 of I. F. amplifier 4 in positive polarity sense, and in adjustable amounts, by connecting the low alternating potential side of input circuit 6 to resistor 20 through a path including resistor 25, lead 24 and slidable tap 23. The condenser 26 grounds the grid side of resistor 25 for alternating current; the latter functioning as a means to suppress all pulsating components in the voltage transmitted over lead 24.

The magnitudes of bias resistors I and 20 are chosen to provide maximum negative bias for grid 8 when receiving an unmodulated carrier. The effective bias on grid 8 is, of course, the algebraic sum of the voltage drops across resistor 1 and that portion of resistor 20 between tap 23 and ground. For example, the maximum effective bias can be 14 volts; adjustment of tap 23 varying the weak modulation gain of amplifier 4. Resistor 20 can be further chosen in magnitude to reduce the maximum efiective bias to 8 volts for 30 to 50% modulation, and 3 volts for 100% modulation. Hence as the modulation intensity increases, the gain of amplifier 4 rises; this follows from the fact that the tap point on resistor 20 becomes increasingly positive with respect to ground as the modulation intensity increases.

To maintain the control of amplifier 4 substantially independent of carrier amplitude variation, the gain of each of the stages feeding amplifier 4 is automatically varied in response to carrier amplitude variations, so as to hold the carrier amplitude at input circuit 6 substantially uniform. The AVC circuit comprises the rectifier diode l3--Il which includes load resistor 30, shunted by I. F. by-pass' condenser 3|, between its electrodes. The I. F. energy is impressed upon anode [3 from the tuned circuit 5, the latter feeding the tuned input circuit 6 of amplifier 4. The condenser 32, of low impedance to I. F. currents, is connected for this purpose between the high alternating potential side of circuit 5 and the anode I3. The anode side of resistor 30 is connected by the lead designated as AVC tothe signal grid circuits of the transmission tubes preceding tube 4. Such AVC connections are too well known to require further description; it is sufficient to point out that as the carrier amplitude at circuit 5 increases, the drop across resistor 30 increases and a greater negative voltage is applied to the controlled tubes preceding amplifier 4. The decrease in gain of the controlled tubes from maximum will be such that the carrier amplitude at input circuit 6 remains substantially uniform.

In operation. the control of amplifier 4 will take place with'minimum distortion. The selectivity to the audio detector, furthermore, will be higher than that to the AVG rectifier; this is a desirable characteristic. Withincrease of modulation intensity, ;as during extremely loud passages of music, the gain of amplifier 4 will be increased. Hence, to the listeners ear the contrast between soft and loud sounds will be heightened. In other words, an expansion of volume range has been secured. Since distortion effects arising by virtue of grid bias variation in I. F. amplifier 4 have substantially no efiect on the fidelity of reproduction, it will be obvious that expansion control is secured in a simple and efiective manner.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organi'zation shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a modulated carrier current receiver, a carrier amplifier, a detector coupled thereto, a rectifier adapted to derive a direct current voltage from the audio voltage component of detected current, means biasing said amplifier to minimum gain in the absence of modulation on received carrier currents, and means for impressing the derived voltage on the amplifier in a polarity sense to reduce the effect of said biasing means.

2. In a modulated carrier current receiver, a carrier amplifier, a detector coupled thereto, a rectifier adapted to derive a direct current voltage from the audio voltage component of detected current, means biasing said amplifier to minimum gain in the absence of modulation on received carrier currents, means for impressing the derived voltage on the amplifier in a polarity sense to reduce the effect of said biasing means, and automatic volume control means for maintaining the carrier amplitude at the amplifier input circuit substantially uniform.

3. In a modulated carrier current receiver, a carrier amplifier, a detector coupled thereto, a rectifier adapted to derive a direct current volt age from the audio voltage component of detected current, means biasing said amplifier to minimum gain in the absence of modulation on received carrier currents, means for impressing the derived voltage on the amplifier in a polarity sense to reduce the efiect of said biasing means, said detector being of the diode type, and said rectifier including an impedance in its space current path to develop said direct current voltage.

4. In a radio receiving system of the type comprising a plurality of cascaded resonant circuits tuned to the same operating signal frequency, at least one amplifier coupling a pair of said cascaded circuits, a rectifier connected to the final resonant circuit to produce an audio voltage from the amplified signal currents, an audio voltage rectifier including a tube having a resistor disposed in its space current path, a second resistor disposed in the space current path of said amplifier, means for connecting the signal input electrode of said amplifier to a predetermined point on said first resistor such that the direct current voltages developed across said resistors are in polarity opposition and the resultant voltage thereof increases in a positive sense as the modulation intensity increases.

5. In a radio receiver which includes a detector having a tuned input circuit, a signal amplifier having its output circuit coupled to said tuned input circuit, said amplifier being provided with a tuned signal input circuit, means providing a substantially high negative bias for the signal input grid of said amplifier in the absence of modulation on the received carrier, means for rectifying the audio voltage output of said detector and producing a direct current voltage which increases in a positive sense as the modulation intensity increases, and means for impressing said last direct current volta e on the amplifier signal grid in a sense to reduce said negative bias and thereby increase the gain of the amplifier as the modulation intensity increases.

6. In a radio receiver which includes a detector having a tuned input circuit, a signal amplifier having its output circuit coupled to said tuned input circuit, said amplifier being provided with a tuned signal input circuit, means providing a negative bias for the signal input grid of said amplifier in the absence of modulation on the received carrier, means for rectifying the audio voltage output of said detector and producing a direct current voltage which increases in a positive sense as the modulation intensity increases, means for impressing said last direct current voltage on the amplifier signal input electrode in a sense to reduce said negative bias and thereby increase the gain of the amplifier as the modulation intensity increases, and means for maintaining the signal carrier amplitude at the input circuit of said amplifier substantially uniform.

7. In a radio receiver which includes a detector having a tuned input circuit, a signal amplifier having its output circuit coupled to said tuned input circuit, said amplifier being provided with a tuned signal input circuit, means providing a substantially high negative bias for the signal input grid of said amplifier in the absence of modulation on the received carrier, means for rectifying the audio voltage output of said detector and producing a direct current voltage which increases in a positive sense as the modulation intensity increases, means for impressing said last direct current voltage on the amplifier signal grid in a sense to reduce said negative bias and thereby increase the gain of the amplifier as the modulation intensity increases, a second rectifier including a load resistor for developing a direct current voltage from signals, means coupling said second rectifier to a point on the receiving system which precedes said amplifier input circuit, and additional means for impressing the direct current voltage output of said second rectifier upon a point in the receiver preceding said amplifier to vary the signal transmission efiiciency through the receiver up to said tuned input circuit thereby to maintain the signal carrier amplitude at the said input circuit substantially uniform.

8. In a receiving system of the superheterodyne type, said system including a second detector circuit which includes a load impedance across which is developed audio voltage derived from the intermediate frequency energy impressed on the detector circuit, an intermediate frequency amplifier having a tuned input circuit and having its output circuit coupled to the input circuit of the detector, at least one intermediate frequency tuned circuit coupled to said amplifier input circuit, a resistor in the space current path of said intermediate frequency amplifier, a rectifier including a resistor in its space current path, means for impressing the audio voltage developed across said detector resistor upon said rectifier whereby direct current voltage developed across the second resistor is dependent in magnitude upon the intensity of said audio voltage, a direct current connection between the signal grid of said intermediate frequency amplifier and a point on said second resistor, the magnitude of said intermediate frequency amplifier resistor being chosen to provide a high negative bias on the amplifier signal grid in the absence of modulation in the carrier, and said point on the rectifier resistor being so chosen that the direct current voltage developed across the rectifier resistor diminishes said amplifier negative bias as the modulation on the carrier increases.

DUDLEY E. FOSTER. 

